Some power supplies used in America are restrained by safety requirements issued by Underwriters Laboratory, and particularly restrained by a UL 1310 Class 2 Standard. In particular, the UL1310 Standard limits an output voltage, an output current and an output power of a power supply that is classified into Class 2, and these limitations shall be strictly met even in the case of single component fault. Taking the output power for example, the power limitation about the UL1310 Class 2 power supply is that each output channel shall not be over 100 W; these channels/outputs may be configured to drive a single light source, such as a solid-state light source (for example, a LED or an OLED), a gas discharge lamp or an incandescent lamp, and the like.
One conventional design scheme is that two voltage converting stages are employed in a power circuit, i.e., a front end stage and an output stage. For example, the front end stage receives a Direct Current (DC) input voltage or an Alternating Current (AC) input voltage, and converts the input voltage into a regulated intermediate voltage. The output stage is coupled to an output terminal of the front end stage, receives the intermediate voltage and uses a buck DC/DC converter to provide a lower desired DC output voltage. However, the power output limitation of the power circuit is mainly implemented through limiting the peak of the switching current of the DC/DC converter. In order to ensure that a power protection point cannot be triggered by mistake under a rated output, the protection can be really triggered in most cases when the output power exceeds the rated power by a great amount; therefore, the control accuracy of the output power is relatively low.